The present invention relates to operational amplifiers, and in particular to a low voltage, low power operational amplifier with rail to rail output.
In integrated circuit technology, smaller design rules have been pursued as a means to reduce chip area, with a corresponding increase in the number of chips that can be produced per wafer and a corresponding decrease in per-chip cost. However, smaller design rules have also decreased the internal signal voltages that can be maintained by a chip. With smaller signal voltage ranges to work with, it becomes increasingly important that integrated circuit signal processing components, including operational amplifiers, are able to make full use of the available signal voltage range. Existing operational amplifiers have been largely unable to handle input voltage swings ranging from the high power supply voltage to the low power supply voltage (i.e. rail to rail), nor have they been able to produce output voltage swings from rail to rail.
Therefore, a need has arisen for an operational amplifier that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for a low power, high speed operational amplifier with the capacity for both input and output signals that swing from rail to rail, or as near to rail to rail as possible.
Accordingly, a novel operational amplifier is disclosed. In one embodiment, the operational amplifier includes an input stage which has a differential amplifier coupled to receive the first and second input voltages. The differential amplifier comprises a first current source providing a first source current to a first node and a second current source providing a second source current to a second node. The differential amplifier provides a first output voltage at the first node and a second output voltage at the second node. The operational amplifier further includes a common mode feedback stage coupled to the first and second nodes. The common mode feedback stage compares an average voltage of the first and second nodes to a reference voltage and changes the first and second source currents in response to a difference between the reference voltage and the average voltage of the first and second nodes. The operational amplifier further includes an output stage coupled to the first and second nodes and an output node. The output stage amplifies voltages at the first and second nodes to provide an output voltage at the output node.
An advantage of the present invention is that the operational amplifier combines fast response with low power consumption and low supply voltage. Another advantage of the present invention is that the output of the operational amplifier swings rail to rail, and the input may swing nearly rail to rail.